Heat Transfer Mechanisms and Tunable Thermal Conductivity Anisotropy in Two-Dimensional Covalent Organic Frameworks with Adsorbed Gases

Two-dimensional covalent organic frameworks (2D COFs) are a novel class of materials that are ideal for gas storage and separation technologies due to their high porosities and large surface areas. In this work we study the heat transfer mechanisms in 2D COFs with the addition of gas adsorbates, demonstrating the remarkably tunable anisotropic response of the phonon thermal conductivity in 2D COFs during gas adsorption. More specifically, our results from atomistic simulations on COF-5/methane systems show that, as the gas density increases, the cross-plane thermal conductivity along the direction of the laminar pores increases, whereas the in-plane thermal conductivity along the 2D sheets is monotonically decreased. We show that a large portion of heat is conducted along the laminar pore channels by the gas molecules colliding with the solid framework and is directly related to the gas diffusivities.